Known techniques of micro-mechanical fabrication are primarily based on technology developed for integrated circuit fabrication. These known techniques can only form geometries of limited complexity. Bulk etching techniques usually suffer from limitations associated with crystallographic angles. H. Seidel, L. Csepregi, A. Heuberger, H. Baumgartel, "Anisotropic Etching of Crystalline Silicon in Alkaline Solutions," JOURNAL 0F ELECTROCHEMICAL SOCIETY, vol. 137 no. 11 pp. 3612-3626, November 1990. Complex geometries can be fabricated by sequentially adding a layer of material, patterning it, and selectively removing portions of each layer. L. S. Fan, Y. C. Tai, and R. S. Muller, "Integrated Movable Micromechanical Structures for Sensors and Actuators", IEEE TRANSACTIONS ON ELECTRONIC DEVICES, vol. 35 no. 6 pp. 724-730, June 1988. N. C. Macdonald, L. Y. Chen, J. J. Yao, Z. L. Zhang, J. A. Mcmillan, and D. C. Thomas, "Selective Chemical Vapor Deposition of Tungsten for Microelectromechanical Structures," SENSORS AND ACTUATORS, vol. 20 pp. 123-133, 1989. However, because material must be removed from each layer after it is patterned, and because it may be necessary to planarize each layer, the number of steps involved may be quite high, thus rendering the cost and time for fabricating a part impractical for industrial purposes. For instance, using conventional methods to fabricate the structure shown in FIG. 1, having an overall width of approximately 550 .mu.m and a thickness of 100 .mu.m, fabrication must proceed in layers. It would be necessary to perform approximately 80 steps of chemical vapor deposition "CVD" patterning, etching, and planarization, if the thickness of each layer is 5 .mu.m.
For fabricating larger mechanical parts (e.g. on the order of 1-100 cm, or, alternatively, having a minimum feature size on the order of 100-200 .mu.m or greater in any dimension), there is currently significant interest in the field of "Desktop Manufacturing." See, M. Fallon, "Desktop Manufacturing Takes You from Art to Part", PLASTICS TECHNOLOGY, February 1989. Most so-called desktop manufacturing processes involve building a part in layers, where the geometry is defined by depositing a layer and then changing the properties of a selected portion of the layer. The term "desktop" is used because an operator at a computer terminal is provided with control over and information regarding every aspect of the manufacturing process, including the operational limits of various process steps.
Typically, multiple layers are added together on top of each other before the excess material is removed from each layer in one final combined step. Such a process can be described as "cumulative." All the layers are sequentially fabricated in the same machine. FIG. 2 schematically illustrates how the structure of FIG. 1 would be formed by such an approach. FIG. 2 includes six panels, showing the progressive steps for forming such a part. In the first panel 202 a layer 214 of material is established. The material is of a composition such that it has properties that can be selectively changed in different regions. Known techniques to selectively change properties include the following: three dimensional printing of a binder material to join layers of powder; laser solidification of photo-polymeriseable liquid polymer; laser sintering of powder; and laser fusing of thin layers of materials.
Three-dimensional printing uses ink-jet printing of a binder material to selectively join layers of powder to form parts. See generally, E. Sachs, M. Cima, J. Cornie, D. Brancazio, and A. Curodeau, "Three Dimensional Printing: Ceramic Tooling and Parts Directly from a CAD Model," National Rapid Prototyping Conference, Dayton, Ohio, June 4-5, 1990. Stereolithography, described in D. Deitz, "Stereolithography Automates Prototyping," MECHANICAL ENGINEERING, February 1990, pp. 34-39, uses an ultra-violet laser to solidify a bath of photopolymerizable liquid polymer on a selective, layer-by-layer basis to create solid polymeric parts. An alternative approach, called "Selective Laser Sintering," uses a laser to sinter areas of loosely compacted powder that is applied layer by layer. See generally, C. Deckard and J. Beaman, "Solid Freeform Fabrication and Selective Powder Sintering," in Proceedings NAMRAC Symposium #15.
After laying the material, the properties of the material in the first layer 214 are selectively changed 204 in specific regions 216. Next another layer 218 of material is applied 206 on top of layer 214. Next the material properties of selected regions of layer 218 are changed 208, as at panel 204.
These steps of layering and selectively altering properties are conducted repeatedly until a full block of material 222 is layered, as shown in panel 210. Captured within the block 222 is a region of altered properties, in the shape of a part desired to be fabricated. At panel 212, the material that makes up the surrounding body of block 222 is removed, such as by firing, and the region made up of the altered material remains, in the shape of the desired part.
To date, it has not been possible to apply such cumulative techniques to the field of micro-manufacturing, i.e. to the manufacture of complex geometries at sizes less than on the order of 100-200 .mu.m. Further, all of the known techniques have drawbacks.
Several other techniques not traditionally connected with selectively altering the properties of a portion of an object are used in connection with the invention. These include: electroless plating; electroplating; sintering; photosensitive metal halide reduction; and photosensitive semiconductor applications.
Electroless plating is a well known process. It is described fully in Pearlstein, F., "Electroless Plating," Modern Electroplating, John Wiley & Sons, New York pp. 710-747 (1974), which is fully incorporated herein by reference. With electroless plating a continuous buildup of metal coating on a substrate arises by immersion in a suitable solution. A chemical reducing agent in the solution supplies electrons for converting metal ions in the solution to the elemental form. However, the reducing action occurs only on a catalytic surface. Thus, in order to continue building up metal on a surface, the metal already deposited must itself be catalytic. The plating occurs in the absence of an applied electric current.
Electroless plating has certain advantages over electroplating, which requires an applied electric current. The substrate need not be an electrical conductor, and regions can be selectively metallized, while other regions are left unmetallized. Power supplies and electrical contacts are not needed. Deposits are often less porous, are produced directly upon non-conductors and have unique chemical, mechanical or magnetic properties.
Electroless plating can be achieved using various metals known to the art, including nickel, copper, gold, silver, platinum, cobalt, palladium, and alloys of one or more of the foregoing.
In contrast to electroless plating, electroplating does require an applied electric potential difference, and thus an electric current between the object to be plated and the solution from which the metal ions are drawn. In electroplating, a substrate upon which metal is to be plated is placed in an electrolytic bath. An electrolytic bath is one having available negative metal ions for plating. By applying an electric potential between the bath and the substrate, with the anode or terminal in contact with the solution, ions are reduced in the solution and are plated onto the surface of the substrate, or the metal surfaces already plated.
A process known as electroforming has also long been known. Electroforming is described fully at F. L. Siegrist, "Have You Considered Electroforming?" METAL PROGRESS, October 1964, pp. 219-230, and F. L. Siegrist, "Electroforming with Nickel: A Versatile Production Technique," METAL PROGRESS, November 1964, pp. 121-130, both of which are incorporated herein by reference. According to this process, a mandrel or master form of predetermined shape, size, accuracy and finish is treated in an electroplating process so that metal is deposited thereon. After the required metal buildup, the mandrel is removed and the metal part remains. The mandrels can be expendable or reusable. Electroformed parts can be made from nickel, copper, silver, iron, gold, rhodium and chromium, depending on engineering requirements, such as electrical or thermal conductivity, optical reflectivity, strength, corrosion or wear resistance.
The reduction of a metal salt, for instance a metal halide, of which silver bromide is exemplary, after exposure to light, is the phenomenon underlying black and white film photography and is well known. A metal halide crystal that is exposed to light is activated, so that when later exposed to a developing solution, which is a chemical reducing agent, the metal is reduced to elemental metal and becomes opaque. Subsequently, the unactivated metal halide is dissolved away with a fixer, such as sodium hyposulphite. This process is well known in the art of photography. A concise description is provided in The Way Things Work, Simon and Shuster, pp. 200-201 (1967), which is incorporated herein by reference.
The semiconductor photo-deposition effect is also well known, and is described fully at M. S. Wrighton, P. T. Wolczanski, and A. B. Ellis, "Photoelectrolysis of Water by Irradiation of Platinized n-Type Semiconducting Metal Oxides," JOURNAL OF SOLID STATE CHEMISTRY, no. 22, pp. 17-19, 1977; Y. C. Kiang, J. R. Moulic, and J. Zahavi, "Metal Silicide Formed by Laser Irradiation of Silicon Chip in Plating Solution," IBM TECHNICAL DISCLOSURE BULLETIN, vol. 26 no. 1, pp. 327, June 1983; and Yu. V. Pleskov and Yu. Ya. Gurevich, SEMICONDUCTOR PHOTOELECTROCHEMISTRY, ch. 7 and 8, Consultant Bureau, New York, 1986 and H. Reiche, W. W. Dunn, and A. J. Bard, "Heterogeneous Photocatalytic and Photosynthetic Deposition of Copper on TiO.sub.2 and WO.sub.3 Powders," JOURNAL OF PHYSICAL CHEMISTRY, vol. 83, no. 17, 1979, pp. 2248, which are all incorporated herein fully by reference. If a body of semiconductor is attached to a body of suitable metal and immersed in an electrolyte solution, a beam of light focused onto the semiconductor, will give rise to a photocell between the semiconductor and the solution. Metal will be reduced from the solution to plate onto the semiconductor.
Another well known semiconductor photoelectric effect, used in connection with photocopying technology is useful in connection with the invention. The surface of the semiconductor nearest to the light source is provided with a large charge, as compared to the surface away from the light source. When the charged surface is struck by light, the resistance within the semiconductor markedly decreases, and current flows within the semiconductor. Charged toner particles in the vicinity of the dark side of the semiconductor are attracted to the conductor. The charge of the surface and particles can be arranged so that the particles are attracted to the locations on the dark side opposite those that were struck by the light or to the locations on the dark side opposite those that were not struck by the light. Other steps not relevant to the invention are used to transfer the charged toner to a recording medium, such as paper, whereupon they are fixed. This process is well known in the art of photocopying. A concise description is provided in The Way Things Work, Simon and Shuster pp. 198-199 (1967), which is incorporated herein by reference.
All of the known processes have drawbacks for micro-manufacturing. Three characteristics are important: 1) geometries must be buildable with high resolution having dimensions as large as 10,000 .mu.m but also having a resolution of less than 1-10 .mu.m; 2) parts must be buildable from metal or ceramic, so that they are mechanically strong or electrically conductive or both; and 3) parts must be buildable relatively quickly. Stereolithography requires the use of expensive photopolymers and can not make parts from either metal or ceramic. Laser sintering is unable to make parts with the required small dimensions and high resolution. VLSI fabrication is a relatively slow process.